The invention concerns a method for detecting the presence of urea deposits in an exhaust line of an automotive vehicle. The invention also concerns a method for eliminating urea deposits in an exhaust line of a vehicle. The invention also concerns an automotive vehicle adapted to perform such methods.
Because of environmental issues, automotive vehicles such as trucks are often equipped with pollution reduction systems. The aim of some of those systems is to reduce the quantity of Nitrogen oxides (NOx), in the exhaust gases of the vehicles. Reduction of NOx (mostly NO and N02) is often realized with a chemical reaction involving ammonia injected in gas exhaust systems.
As storage and carriage of ammonia in an automotive vehicle is not safe, ammonia can be brought by the injection of urea, or of an aqueous solution thereof, in order for it to decompose into ammonia and react, in a selective catalytic reduction system (SCR), with NOx to produce ideally nitrogen and water.
In order to obtain ammonia from urea, several factors need to be combined, and especially temperature must be high enough for the decomposition reaction to work properly. If the temperature is not high enough, or if other factors are not present, the decomposition reaction may produce solid compounds which may accumulate in the gas exhaust pipes in the form of solid deposits. These solid deposits can obstruct the pipes and induce performance and security issues. For instance, the deposits in the exhaust pipes can reduce the power of the vehicle, or lead to over-pressures in the exhaust line. Those deposits need to be cleaned regularly. In the following, those solid deposits will be referred to as urea deposits although their exact composition may vary and may not comprise only urea, but also intermediate products of the decomposition reaction of urea into ammonia.
It is known that the urea solid deposits can be decomposed to ammonia when exposed to a temperature exceeding a certain threshold, typically around 350° C. Such a reaction will be referred herein as the thermal elimination of the urea solid deposit. Automotive combustion engines produce high temperature exhaust gases, but the temperature of the exhaust gases may vary considerably depending on the engine load. Moreover, temperature of the exhaust gases will inevitably decrease along the exhaust line due to their flowing through various devices such as turbines or particle filters, or simply due to heat exchange between the exhaust line and its environment. Therefore, especially if the SCR system is located downstream in the exhaust line, the potential urea deposits may be exposed to a suitable temperature for their thermal elimination only during a fraction of the operating time of the engine. During that fraction of time, no substantial deposit will be formed, and previously formed deposit will be “naturally” thermally eliminated. On the other hand, outside of that fraction of time, not only urea deposits will not be eliminated, but further deposit may be formed.
Various techniques are known to “artificially” enhance the exhaust gas temperatures of an engine, for example by modifying the engine operating conditions or by heating the exhaust gases with a heating device. Unfortunately, these techniques all have in common an increased energy consumption, and particularly a fuel over-consumption. Known methods execute a forced thermal elimination of the urea deposits by artificially increasing the exhaust gases temperature at regular intervals, and for a predetermined period of time, irrespective of whether urea deposits have formed in the exhaust line, and irrespective of the amount of deposit to be eliminated. For instance, such methods initiate a predefined forced thermal elimination period each time a given distance has been covered by the vehicle since the last forced elimination.
In order to avoid a fuel over-consumption, some methods detect urea deposits in the exhaust pipes and initiate the regeneration reaction only when needed. It is known, for instance from US-A-200810271440, to measure pressure variations in exhaust pipes and/or SCR system to detect urea deposits. Results given by this solution lack precision and are difficult to exploit.
Thus, known techniques of detection do not permit to precisely conclude about presence or not of urea deposits in exhaust pipes at a given time.
This invention aims, according to an aspect thereof, at proposing a new method for detecting and eliminating urea deposits in an exhaust line of an automotive vehicle, which permits to determine precisely if urea deposits are present in exhaust pipes, in order to begin the cleaning of the pipes only when needed, and to stop it when the cleaning is finished.
To this end, an aspect of the invention concerns a method for detecting urea deposits in an exhaust line of an automotive vehicle, equipped with an internal combustion engine, a selective catalytic reduction system, mounted on the exhaust line, adapted to convert NOx produced by the internal combustion engine, and an injection system adapted to inject urea in the exhaust line, upstream the selective catalytic reduction system. This method is characterized in that it comprises the following steps:
a) determination if an exhaust gases temperature is reached,
b) if the result of determination made at step a) is positive, stoppage of urea injection,
c) determination of the quantity of NOx in the exhaust gases on the outlet of the selective catalytic reduction system,
d) comparison between the quantity of NOx determined at step c) and a theoretical quantity or a measured quantity of NOx produced by the internal combustion engine,
e) if step d) shows that said quantities are different, considering that urea deposits are present in the exhaust line.
Thanks to an aspect of the invention, urea deposits in the exhaust pipes are detected by the difference between the quantity of NOx downstream the internal combustion engine and downstream the selective catalyst reduction system. In fact, regeneration reaction of urea deposits is used to detect presence of deposits. The precision of this detection permits to start a cleaning procedure only when needed, avoiding fuel over-consumption in the case cleaning is done by burning deposits.
According to further aspects of the invention which are advantageous but not compulsory, such a method can include the following features:
The theoretical quantity of NOx produced by the internal combustion engine is determined on the basis of the operating conditions of said internal combustion engine.
The quantity of NOx produced by the internal combustion engine is determined through the use of a sensor in an exhaust pipe of said exhaust line upstream of said selective catalytic reduction system.
The method is initiated only if a given distance has been covered and/or a given time period has elapsed since the last detection of presence of urea deposits.
The quantity of NOx determined at step c) is a mean value computed over a given period of time.
A given quantity of ammonia or ammonia precursor is stored, prior to step b) in a portion of the selective catalytic reduction system, then the quantity of NOx is determined at step c) after release of the given quantity of ammonia or ammonia precursor in the selective catalytic reduction system.
The method comprises, prior to step a), a further step of:
f) altering the operating conditions of the internal combustion engine or otherwise heating the exhaust gases to raise the exhaust gases temperature.
At step d), comparison is made by comparing a NOx conversion rate value (Rm) to a minimum threshold.
The invention also concerns, according to an aspect thereof, a method for eliminating urea deposits in an exhaust line of an automotive vehicle, said method implementing the aforementioned method for detecting urea deposits. This method also comprises a step of:
f) if at step e) one can consider that urea deposits are present in the exhaust line, cleaning of the exhaust line.
According to further aspects of the invention which are advantageous but not compulsory, such a method for eliminating urea deposits may incorporate one or several of the following features:
The detection method is performed during the cleaning of the exhaust line.
If at step e) one cannot consider that urea deposits are present in the exhaust line, the cleaning of the exhaust line is stopped and the urea injection system is reactivated.
The invention also concerns an automotive vehicle with which the above-mentioned method can be implemented. More precisely, the invention concerns an automotive vehicle equipped with a selective catalytic reduction system mounted on an exhaust line of the vehicle and adapted to convert NOx produced by an internal combustion engine of the vehicle, an injection system adapted to inject urea in the exhaust line, upstream the selective catalytic reduction system, a NOx sensor mounted in the exhaust line, downstream the selective catalytic reduction system, and an exhaust gases temperature sensor upstream the selective catalytic reduction system. This vehicle is characterized in that it comprises also means adapted to stop operation of the urea injection system, to compare a quantity of NOx measured by the NOx sensor to a theoretical NOx quantity or a measured NOx quantity produced by the internal combustion engine, and to perform cleaning of the exhaust line on the basis of the result of the comparison between said quantities.
According to further aspects of the invention which are advantageous but not compulsory, such a vehicle may include the following features:
The means to stop operation of the urea injection system, the means to compare and the means to perform cleaning are included in an electronic control unit.
The electronic control unit is adapted to compute a theoretical value of the quantity of NOx produced by the internal combustion engine, on the basis of the operating conditions of the engine.
The electronic control unit is adapted to control the operating conditions of the internal combustion engine.
The vehicle comprises means to store ammonia or an ammonia precursor in the selective catalytic reduction system.